CBSE-XI-Physics
22: Photometry
- #4An electric bulb is hanging over a table at a height of 1 m above it. The illuminance on the table directly below the bulb is 40 lux. The illuminance at a point on the table 1 m away from the first point will be about
(a) 10 lux
(b) 14 lux
(c) 20 lux
(d) 28 luxdigAnsr: bAns : Correct option
(b)
`` \,\mathrm{\,Here\,},``
`` \,\mathrm{\,r\,}=\sqrt{2}``
`` \,\mathrm{\,tan\theta \,}=\frac{\,\mathrm{\,BC\,}}{\,\mathrm{\,AB\,}}=1``
`` {\,\mathrm{\,I\,}}_{\,\mathrm{\,o\,}}=40\,\mathrm{\,lux\,}``
`` \,\mathrm{\,\theta \,}={\,\mathrm{\,tan\,}}^{-1}\left(1\right)={45}^{0}``
`` \,\mathrm{\,The\,}\,\mathrm{\,illuminance\,}\,\mathrm{\,is\,}\,\mathrm{\,given\,}\,\mathrm{\,by\,}``
`` \,\mathrm{\,E\,}=\frac{{\,\mathrm{\,I\,}}_{\,\mathrm{\,o\,}}\,\mathrm{\,cos\theta \,}}{{\,\mathrm{\,r\,}}^{2}}``
`` =\frac{40\times \,\mathrm{\,cos\,}\left({45}^{0}\right)}{{\left(\sqrt{2}\right)}^{2}}``
`` =14\,\mathrm{\,lux\,}``
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